We can study the skewing of the energy of the beam compared to the isotropic case (Δ1 angle which is the difference between the values of ψ and θ1 angles as defined in Fig. 13), but usually skewing is expressed by the Δ2 angle which is defined as the angle between the wave normal and the wave’s energy flow direction (difference between the values of ψ and θ2 angles as defined in Fig. 13).